1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to an MOS transistor.
2. Description of the Related Art
In a conventional MOS integrated circuit, a thick insulating film is formed on a so-called field region between elements in order to eliminate an insulation failure due to a parasitic channel and reduce the parasitic capacitance of wiring.
As a typical method of forming such a film, the selective oxidation technique is employed In this method, a silicon nitride film is formed on a flat surface of a silicon wafer through a thin silicon oxide film, and the silicon wafer is oxidized using the silicon nitride film as a mask, thereby selectively forming a thick oxide film
According to another known technique, a groove is formed in the field region of a wafer by etching, and a silicon oxide film or the like is flatly buried in the groove by a CVD method.
Even these conventional techniques, however, have various problems to be solved.
For example, when an MOS transistor incorporated in an LSI is to be operated, the substrate potential may be changed because of noise, or the source potential may be changed due to charging/discharging of a load capacity or the like. As a result, the threshold voltage of the MOS transistor is shifted. If the source voltage of, e.g., an n-channel transistor is changed in the positive direction, an equivalent negative substrate potential is practically applied to the n-channel transistor.
Such a problem degrades the current supply capacity of an MOS transistor or decreases the switching speed, thus adversely affecting the circuit operation.
Therefore, a demand has arisen for a device having an excellent cutoff characteristic, i.e., having a drain current which rises steeply in response to changes in gate voltage
As described above, in the conventional techniques, a threshold voltage is easily changed in response to changes in substrate bias or the like, and a device having an excellent cutoff characteristic cannot be obtained.